computer graphics23

Color

Color Vision

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Review of last week

Color Vision

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Review of color • Spectrum • Cone sensitivity function • Metamers – same color, different spectrum

• Opponent – black-white, blue-yellow, red-green

• Color spaces – Linear algebra – Problem with negative values – Standard: CIE XYZ

• Perceptually uniform color space – CIE Lab (non-linear wrt XYZ) Color Vision

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CIE-Lab (a.k.a. CIE L*a*b*)

Image adapted from Wyszecki, G., and W. S. Stiles,Color science: Concepts and methods, quantitative data and formulae.

Wiley-Interscience; 2nd Edition. July 2000. ISBN: 0471399183

Color Vision

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Perceptually Uniform Space: MacAdam • • •

In color space CIE-XYZ, the perceived distance between colors is not equal everywhere In perceptually uniform color space, Euclidean distances reflect perceived differences between colors MacAdam ellipses (areas of unperceivable differences) become circles

510

490

.05

520

530

.10 .15

540 550 560 .65 .60 .55 .50 .45

.20

.25.30 .25

Spectrum locus 570 580 590 .35

600

.60 .40 .45 .50 .55

610620 650 .65 .70 700 nm

.20 .15

480

y x

470 460

.10 .05

ple Pur

line

450 400 nm

Image adapted from: Wyszecki, G. and W. S. Stiles. Color science: Concepts and Methods, Quantitative Data and Formulae. Wiley-Interscience, 2nd ed. July 2000. ISBN: 0471399183.

Color Vision

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Hue Saturation Value • • • •

Value: from black to white Hue: dominant color (red, orange, etc) Saturation: from gray to vivid color HSV double cone

value saturation

hue saturation

Color Vision

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Hue Saturation Value • One interpretation in spectrum space • Not the only one because of metamerism • Dominant wavelength (hue) • Intensity • Purity (saturation)

Color Vision

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CIE color space • Match color at some point A • A is mix of white C, spectral B! • What is dominant wavelength of A? • What is excitation purity (%) of A? – Move along AC/BC

y 0.8 0.7

520 540

510

B 560

0.6 0.5 0.4 0.3

A

500 D 490

0.2

600 C

480

0.1

400 0.1

580

F

C

E

700

G

0.2 0.3 0.4 0.5 0.6 0.7

x

Image adapted from: Hunt, R. W. G. The Reproduction of Colour. John Wiley & Sons Incorporated. September 2004. ISBN: 0-470-02425-9.

Color Vision

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Plan • Color Vision • Color spaces • Color effects – Definitions – Spatial sensitivity – Color illusion and color appearance

• Producing color

Color Vision

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Color terms (Fairchild 1998 • • • • • •

Color Hue Brightness vs. lightness Colorfulness and Chroma Saturation Unrelated and related colors

Color Vision

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Color – chromatic and achromatic content. This attribute can be described by chromatic color names such as yellow, orange, brown, red, pink, green, blue, purple, etc., or by achromatic color names such as white, gray, black, etc., and qualified by bright, dim, light, dark, etc., or by combinations of such names. – Note: Perceived color depends on the spectral distribution of the color stimulus, on the size, shape, structure, and surround of the stimulus area, on the state of adaptation of the observer's visual system, and on the observer's experience of the prevailing and similar situations of observations.

Color Vision

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Related and Unrelated Colors • Unrelated Color – Color perceived to belong to an area or object seen in isolation from other colors.

• Related Color – Color perceived to belong to an area or object seen in relation to other colors.

Color Vision

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Hue • Hue – Attribute of a visual sensation according to which an area appears be similar to one of the perceived colors: red, yellow, green, and blue, or to a combination of two of them.

• Achromatic Color – Perceived color devoid of hue.

• Chromatic Color – Perceived color possessing a hue.

Color Vision

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Brightness vs. Lightness • Brightness – Attribute of a visual sensation according to which an area appears to emit more or less light.

• Lightness: – The brightness of an area judged relative to the brightness of a similarly illuminated area that appears to be white or highly transmitting.

Color Vision

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Colorfulness & Chroma • Colorfulness – Attribute of a visual sensation according to which the perceived color of an area appears to be more or less chromatic.

• Chroma: – Colorfulness of an area judged as a proportion of the brightness of a similarly illuminated area that appears white or highly transmitting.

Color Vision

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Saturation – Colorfulness of an area judged in proportion to its brightness.

Color Vision

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Plan • Color Vision • Color spaces • Color effects – Definitions – Spatial sensitivity – Color illusion and color appearance

• Producing color

Color Vision

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Cornsweet illusion • Two opposite gradients • We judge only the contrast at the edge

Color Vision

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Contrast processing • Receptors are wired to other neurons • Center-surround organization • Sensitive mostly to local contrast - - - + - + ++ ++ - --

Color Vision

+ + + + -- + + + --- + + + ++

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Land Retinex and local contrast

Color Vision

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e Color Vision

d

c

b

a

Perceived Brightness (e.g. scale)

• Contrast is enhanced at region boundaries

Actual Intensity (e.g. lumina)

Mach Bands light

dark

a

b

c

d

e

a

b

c

d

e

light

dark 21

Hermann Grid --- - - -- - - - - ----- + + -- --- +++ ---- - - - - -- - - - -

-- -- - --- - - - -+++ -- -- + --- - +--- - -- - -- - - -- -

Color Vision

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Brightness vs. lightness • Brightness: subjective amount of light • Lightness: how “white”

The white cells in shadow are as dark as the black illuminated cells Color Vision

(Courtesy of The Department of Brain and Cognitive Sciences. Used with permission.)

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Brightness vs. lightness • Brightness: subjective amount of light • Lightness: how “white”

The white cells in shadow are as dark as the black illuminated cells Color Vision

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Brightness vs. lightness • Brightness: subjective amount of light • Lightness: how “white”

The white cells in shadow are as dark as the black illuminated cells Color Vision

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Lightness and transparency

Color Vision

(Courtesy of The Department of Brain and Cognitive Sciences. Used with permission.)

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Opponents and image compression • JPG, MPG • Color opponents instead of RGB • Compress color more than luminance Color Vision

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Contrast Sensitivity • Sine Wave grating • What contrast is necessary to make the grating visible?

(Image removed due to copyright considerations.)

Color Vision

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Contrast Sensitivity Function (CSF)

(Image removed due to copyright considerations.)

Color Vision

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Contrast Sensitivity Function (CSF)

Contrast Sensitivity

100 Luminance

10 1

Red-Green

0.1 0.01

Blue-Yellow

-1

0

1

2

Log Spatial Frequency (cpd) Spatial contrast sensitivity functions for luminance and chromatic contrast.

Image adapted from: Mark D. Fairchild. Color Appearance Models. Prentice Hall PTR, November 1997. ASIN: 0201634643

Color Vision

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JPEG Compression • Perform DCT to work in frequency space – Local DCT, 8x8 blocks

• Use CSF for quantization (more bits for sensitivity with more contrast) • Other usual coding tricks

Color Vision

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Plan • Color Vision • Color spaces • Color effects – Definitions – Spatial sensitivity – Color illusion and color appearance

• Producing color

Color Vision

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Color constancy • Chromaticity of light sources vary • Chromatic adaptation – Similar to white balance on camera – Different films, filters

Color Vision

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Chromatic adaptation • Von Kries adaptation • Different gain control on L, M, S • Similar to white balance on camera 0.75, 1, 1

0.75, 1, 1

Gain control: *1.33, *1, *1 0.2, 1, 0.2 Color Vision

0.15, 1, 0.2

0.2, 1, 0.2 34

Crispening • Increased sensitivity

Color Vision

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Stevens effect • Stevens effect

Relative Brightness

– Perceived contrast increases with luminance

1.0

0.1

L (Dark)

Relative Luminance L (63 dB)

1.0

L (79 dB)

L (97 dB)

Color Vision

Image adapted from: Fairchild, Mark D. Color Appearance Models. Prentice Hall PTR. November 1997.

ASIN: 0201634643.

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Hunt and Stevens effect • Stevens effect – Perceived contrast increases with luminance

• Bartleson-Breneman effect – Image contrast changes with surround – A dark surround decreases contrast (make the black of the image look less deep) – Important for movies

• Hunt effect – Colorfulness increases with luminance

• Hence the need for gamma correction (see later) Color Vision

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Color appearance models • Predict the appearance of a color depending on – Objective stimulus – Surrounding, context

Color Vision

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Plan • • • •

Color Vision Color spaces Color effects Producing color

Color Vision

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Color synthesis Additive red, green, blue

Add light (CRT, video projector) Color Vision

Subtractive cyan, magenta, yellow

Remove light (e.g. filter) printer, photos 40

Color synthesis: a wrong example Additive

Subtractive

red, green, blue

WRONG

Color Vision

cyan, magenta, yellow

RIGHT 41

Device Color Models(Subtractive) Start with white, remove energy (e.g.,w/ ink, filters) Example: CMY color printer Cyan ink absorbs red light Magenta ink absorbs green light Yellow ink absorbs blue light C+M+Y absorbs all light: Black!

RGB => CMY conversion Some digital cameras use CMY Color Vision

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CMYK CMY model plus black ink (K) Saves ink Higher-quality black Increases gamut

Conversion not completely trivial

Color Vision

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The infamous gamma curve A gamma curve x->xγ is used for many reasons: • CRT response – The relation between voltage and intensity is non-linear

• Color quantization – We do not want a linear color resolution – More resolution for darker tones – Because we are sensitive to intensity ratios

• Perceptual effect – We perceive colors in darker environment less vivid – Hunt and Stevens effect

• Contrast reduction – Keep some contrast in the highlights Color Vision

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Gamut •

• •

Every device with three primaries can produce only colors inside some (approx.) triangle – Convexity!

This set is called a color gamut – (Why can’t RGB can’t give all visible colors?)

Usually, nonlinearities warp the triangle y – Also, gamut varies with luminance 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1

520 540 560

510

o

500

580 600

490 480

y

x

700

400 0.1

0.2 0.3 0.4 0.5 0.6 0.7

x

Image adapted from: R. W. G. Hunt. The Reproduction of Colour. John Wiley & Sons, Incorporated, September 2004. ISBN: 0-470-02425-9.

Color Vision

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Gamut Mapping • Color gamut of

•

different processes may be different (e.g.

CRT display and 4color printing process) Need to map one 3D color space into another

CIE-Lab Perceptually-uniform Color space

Typical CRT gamut 4-color printing gamut

Color Vision

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Gamut Mapping Typical CRT

4-color CMYK printing

gamut

gamut

b*

a*

Gamut mapping is a morphing of 3D color space according to adopted scheme

Color Vision

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ICC standard • Every device has a different color response (gamut, spectrum) • Picture file should store this color profile • ICC standard • Unfortunately not spread enough

Color Vision

http://www.color.org/

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Selected Bibliography Vision Science

by Stephen E. Palmer MIT Press; ISBN: 0262161834 760 pages (May 7, 1999)

Billmeyer and Saltzman's Principles of Color Technology, 3rd Edition

by Roy S. Berns, Fred W. Billmeyer, Max Saltzman Wiley-Interscience; ISBN: 047119459X 304 pages 3 edition (March 31, 2000)

Vision and Art : The Biology of Seeing

by Margaret Livingstone, David H. Hubel Harry N Abrams; ISBN: 0810904063 208 pages (May 2002)

Color Vision

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Selected Bibliography The Reproduction of Color

by R. W. G. Hunt Fountain Press, 1995

Color Appearance Models

by Mark Fairchild Addison Wesley, 1998

Color Vision

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